1. Field of the Invention
The present invention relates to a half tone recording method applied to an optical image recorder such as a laser printer, a digital copying machine, a laser facsimile, etc., and a light source device for writing used in this half tone recording method.
2. Description of the Related Art
Recently, plain paper can be used and a high quality image can be obtained at a high speed in a laser printer constructed by combining an electrophotographic technique and a laser scanning technique with each other. Therefore, such a laser printer has been rapidly spread as an output device of a computer, a digital copying machine, etc.
In such a situation, to obtain a higher quality image, it is effective to use a recording system for making resolution and gradient consistent with each other by a one-dot multivalued recording system.
The multivalued recording system used in the laser printer is generally divided into a light intensity modulating system and a pulse width modulating system. In the pulse width modulating system, the recording operation of an image can be comparatively stably performed irrespective of external changing factors since a binary recording operation is approximately performed.
FIG. 1 shows the relation between an exposure amount and an image density in electrophotographic recording. An electrophotographic device has unsaturated and saturated regions with respect to the image density. In the unsaturated region, the image density is increased in accordance with the exposure amount until an exposure amount E.sub.0. In the saturated region, the image density is saturated in a range in which the exposure amount is equal to or greater than the exposure amount E.sub.0. An intermediate exposure region of a photosensitive body as the unsaturated region is used in a system for modulating the light intensity of a semiconductor laser so that a high accuracy in exposure energy control is required. A control technique for obtaining such a high accuracy can be realized by forming a high speed photoelectric negative feedback loop. Namely, 256 gradations are easily realized by a pixel clock signal having 20 MHz using this control technique.
However, in the pulse width modulating system, a time pitch for changing pulse widths is significantly decreased as a laser scanning speed is increased (as the frequency of a writing pixel clock signal is increased). For example, in the case of the pixel clock signal having 20 MHz, a time pitch of about 0.2 nanoseconds is required when the number of gradations represented by one dot is set to 256. Accordingly, the pulse width modulating system has serious problems concerning accuracy and cost.
When an image is formed by an electrophotographic process using the light intensity modulating system, the rotational speed of a recording medium as the photosensitive body is changed and the image density is also changed by the inclination of a polygonal face. Further, no surface potential of the photosensitive body has a steep distribution in a low density section. Therefore, the reproducibility of dots is reduced. Each of FIGS. 2a and 2b shows an exposure energy distribution when an image is recorded by pulse width modulation and light intensity modulation at the same image density. FIGS. 2a and 2b respectively show cases of the pulse width modulation and the light intensity modulation in which 400 dpi is set and an exposure beam diameter is set to 40 .mu.m in a main scanning direction and 80 .mu.m in a sub-scanning direction. In the pulse width modulating system, the saturated region has a large area and gradation about area is approximately obtained. In contrast to this, the unsaturated region has a large area in the light intensity modulating system. This is because the image density tends to be changed by a change in overlapping degree of exposure energy between adjacent scanning lines caused by a change in scanning line pitch.
A half tone recording system provided by combining the light intensity modulating system with the pulse width modulating system is proposed to improve the above disadvantages of these modulating systems. In this half tone recording system, a light emitting pulse width of a light source can be freely selected in a divisional pulse unit (.DELTA.T.sub.1, .DELTA.T.sub.2, - - - ) obtained by dividing a pulse width T with respect to one pixel. Further, light intensity of the light source can be freely selected at plural values such as light intensity values of m kinds (P.sub.1, P.sub.2, - - - , P.sub.m in a low order of the light intensity) with respect to such divisional pulses. After the light intensity about a certain divisional pulse is increased from the minimum light intensity P.sub.1 to the maximum light intensity P.sub.m, the light intensity about a divisional pulse adjacent to this certain divisional pulse in the main scanning direction is increased from the minimum light intensity P.sub.1 to the maximum light intensity P.sub.m. Such processing is repeatedly performed sequentially. Thus, an image is recorded by selecting the light emitting pulse width and the light intensity in accordance with an increase in density level of image information.
However, in such a combinatorial half tone recording system, a toner particle diameter is large in comparison with one dot in the present electrophotographic process and a toner image is disturbed at a transfer time. Therefore, an image having a very low quality is obtained when a one-dot multivalued recording operation is performed.